Minimum sum

Given an array Arr of size N such that each element is from the range 0 to 9. Find the minimum possible sum of two numbers formed using the elements of the array. All digits in the given array must be used to form the two numbers.

• Time : O(nlogn)
• Space : O(1)

class Solution{
public:
    string solve(int arr[], int n) {
        string res = "";
        int carry = 0;
        sort(arr, arr + n);
        
        for(int i = n - 1; i >= 0; i -= 2) {
            int sum = carry + arr[i] + (i ? arr[i - 1] : 0);
            res += string(1, (sum % 10) + '0');
            carry = sum / 10;
        }
        
        res += string(1, carry + '0');
        while(!res.empty() and res.back() == '0') res.pop_back();
        
        reverse(begin(res), end(res));
        return res;
    }
};